A magnet falling in a metal tube

by @ulaulaman http://youtu.be/keMpUaoA3Tg about #magnet #school #physics #experiments #magnetic_field

Sometimes teach physics, and a couple of years ago the colleague of the educational laboratory proposed to the students a simple experiment: a magnet that descends along a metal tube⁽¹⁾ does not fall at the same speed of the same magnet that we throw from the same height, for example, inside a not metal tube. The magnet inside the metal tube will drop almost as if it remained in suspension, swinging a little to the right a little to the left, in a manner apparently inexplicable, but just let us allow to the students, exceeded the initial amazement, to think above the phenomena, even without too much knowledge, and they realize that in some way that the behavior of the magnet inside the tube is due to an interaction between the two objects that goes beyond the simple direct contact.
The secret behind the behavior of the magnet is the so-called Lenz's law:

An induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.

In the case of the falling magnet, the induced emf is the one that starts to flow in the metal tube and this one generates a magnetic field that opposes to the magnetic field of the falling magnet, making thus slow, since its opposition goes in the opposite direction to the gravity.
However it is possible to verify the effect both quantitatively and qualitatively: the experiment, in fact, is quite old and known to be examined more or less from every angle and is quite simple to accomplish. Indeed the experiment could be proposed also in a high school (I did!), not only for the experimental part of the curricula, but also for the theoretical part. For example an interesting experiment by a group of Venezuelan physicists⁽²⁾ could be useful to intreoduce the differential calculus:

Another interesting paper is dued by a Canadian group⁽³⁾: the currents generated by the falling magnet inside the tube are colled eddy currents, or Foucault currents after their discovery by Jean Bernard Léon Foucault in 1851. This type of current aren't generated only by a moving magnetic field, but also by a variable magnetic field and by alternating current. Among the many applications, we include the induction cookers, the brakes of the train and the levitation of superconductors.

---

Further reading:
Eddy currents
If Copper is Not Magnetic, How Can it Affect a Falling Magnet? Lenz's Law

Video
Magnet in a copper pipe | Eddy currents | Eddy current tubes | Copper pipe magnet

---

(1) Magnet traveling through a conducting pipe: a variation on the analytical approach
(2) Donoso G., Ladera C.L. & Martín P. (2011). Damped fall of magnets inside a conducting pipe, American Journal of Physics, 79 (2) 193. DOI: 10.1119/1.3531964 (pdf: archive.org | 4shared)
(3) MacLatchy C.S., Backman P. & Bogan L. (1993). A quantitative magnetic braking experiment, American Journal of Physics, 61 (12) 1096-1101. DOI: 10.1119/1.17356 (pdf)